Objective: Increasing use of fertility drugs is considered the primary cause for the recent increase in dizygotic twinning in developed countries. However, dizygotic twinning has also been related to obesity in foreign populations. We sought to confirm this relationship in U.S. pregnancies, which predated widespread use of fertility drugs.
Methods: We analyzed 51,783 pregnancies (561 twin) in the Collaborative Perinatal Project, which took place at 12 hospitals in the United States from 1959 to 1966. The occurrence of twinning was compared according to maternal self-reported prepregnant body mass index (BMI) of less than 20, 20-24.99, 25-29.99, and 30 kg/m(2) or greater, before and after adjustment for confounding factors.
Results: There was a statistically significant trend for increased risk of total twinning with increasing BMI (P < .001). The odds of monozygous twinning were not significantly related to BMI, but the odds of dizygous twinning were significantly related to increased BMI. After adjusting for maternal race, age, parity, and height, the odds of dizygous twinning were still significantly elevated among women with a BMI of 30 or more, and the trend for increasing risk of dizygous twinning with increasing BMI was significant (P = .001). The trend for increased twinning with increasing height was also significant. Women in the tallest quartile of height had a significantly increased odds ratio for dizygous twin pregnancies, although not of the same magnitude as women with BMI over 30.
Conclusion: We confirmed the association of maternal weight and height with dizygotic twinning in a U.S. population among which fertility drugs were not a factor.